This application is to request support for a Keystone Symposia meeting entitled "Regulatory T Cells", organized by Fiona M. Powrie and Shimon Sakaguchi, which will be held in Keystone, Colorado from March 1 - 6, 2009. Over the past few years Regulatory T cells have taken center stage in the field of immunoregulation. Several types of regulatory T cell have been described on the basis of their origin, generation and mechanism of action, with two main subsets identified: naturally occurring FOXP3+ regulatory T cells and inducible regulatory T cells. All types of regulatory T cell, by virtue of their capacity to control the intensity of effector responses, have been shown to play a major role in the control of immune responses at large. This meeting will highlight recent developments in our understanding of the molecular basis for the differentiation of naturally occurring or inducible regulatory T cells. The second goal is to discuss information on the targets, mechanism of action, recruitment and homeostasis of regulatory T cells. The third goal is to discuss recent advances in our understanding of the conditions favoring the emergence and regulation of these populations and to discuss how these findings are translated in clinical trials. Regulatory T cells (Tregs) have become the topic of intense investigation in the last decade primarily because of their essential roles in immune homeostasis. Tregs are now considered to be a good target for therapeutic interventions such as provoking tumor immunity in cancer patients, treating autoimmune disease such as type 1 diabetes and preventing graft-versus-host-host disease after bone marrow transplantation. This meeting will provide updates and critical discussions regarding our rapidly advancing knowledge on fundamental aspects of Treg biology and pathology and their potential clinical utility.